docs: link to migrating to 6 in 6.x docs

Author: vkarpov15

docs/layout.pug

@@ -143,7 +143,7 @@ html(lang='en')
               li.pure-menu-item.sub-item
                 a.pure-menu-link(href=`${versions.versionedPath}/docs/api/virtualtype.html`, class=outputUrl === `${versions.versionedPath}/docs/api/virtualtype.html` ? 'selected' : '') VirtualType
               li.pure-menu-item
-                a.pure-menu-link(href=`${versions.versionedPath}/docs/migrating_to_7.html`, class=outputUrl === `${versions.versionedPath}/docs/migrating_to_7.html` ? 'selected' : '') Migration Guide
+                a.pure-menu-link(href=`${versions.versionedPath}/docs/migrating_to_6.html`, class=outputUrl === `${versions.versionedPath}/docs/migrating_to_7.html` ? 'selected' : '') Migration Guide
               li.pure-menu-item
                 a.pure-menu-link(href=`${versions.versionedPath}/docs/compatibility.html`, class=outputUrl === `${versions.versionedPath}/docs/compatibility.html` ? 'selected' : '') Version Compatibility
               li.pure-menu-item